This invention relates to devices and methods for determining whether or not a battery is fully charged.
Batteries have numerous applications. Some batteries are used in electrical generating stations, electrical switching stations and other such applications to provide backup power. Commercial and industrial facilities use batteries to power emergency systems in the event normal AC power is interrupted. Electric vehicles using batteries are presently being developed by automobile manufacturers. In all these and many other applications, it is often important to know whether or not a battery has reached a fully charged state. It also is important to detect shorted cells early.
One existing method for determining the charge state of a battery relies on the use of a hydrometer. However, such a reading may be inconvenient and may not indicate a full charge until weeks after the battery is in fact fully charged. Other methods measure battery open circuit voltage to determine whether a battery is fully charged. Such methods, such as the ones described in U.S. Pat. No. 4,937,528 to Palanisamy and U.S. Pat. No. 4,423,379 to Jacobs, et. al. specifically require dissipation of the polarization voltage before measuring open circuit voltage for full charge. However, open circuit voltage varies with load, and such methods may be inaccurate if a load is connected to the battery during the measurement. Existing methods such as disclosed by Palanisamy in U.S. Pat. No. 4,937,528 rely on low open circuit voltage and/or inflections in a dV/dI curve to detect a shorted cell. However, open circuit voltage is not always a reliable early indicator of a shorted cell.
Shorted cells also are difficult to detect early. It has been observed that shorted cells initially have high resistance shorts (on the order of 1 ohm). Because battery resistance is small (on the order of 0.1 to 10 milliohms) a change in battery resistance due to this "parallel" short is difficult to detect by existing means, for example, using the battery monitor described in U.S. Pat. No. 4,697,134 to Burkum and Gabriel. It is only when the battery is shorted with a low resistance short and can not be charged that a shorted cell becomes evident. However, in many applications it is important to detect a shorted cell prior to this point.
Contrary to some previous methods, observation of the polarization voltage can indicate that a battery or individual cells of a battery are fully charged. Likewise, absence of the polarization voltage in a cell where polarization voltage is present in other cells in the battery can indicate that the cell is less than fully charged and therefore shorted. The polarization voltage is a counter electromotive force caused by a change in the density of the electrolyte in the pores of the battery plates. During charging the polarization voltage increases and eventually stabilizes, at which time the battery is fully charged and gassing. This can occur several weeks before a hydrometer test would indicate that a battery is fully charged. It may be many hours before the polarization voltage stabilizes.
An apparatus that could quickly and easily detect the stabilized polarization voltage would be extremely beneficial. Under certain circumstances, such an apparatus also could indicate the presence of a shorted cell. It also would be valuable to perform this test without disconnecting the battery from its electrical load.
It is therefore an object of this invention to determine when an individual cell of a battery has reached full charge.
It is another object of this invention to determine whether an individual cell of a battery is shorted.
It is a further object of this invention to determine if a cell of a battery connected to a battery charger and an electrical load is fully charged.
It is another object of this invention to determine if a cell of a battery connected to a battery charger and an electrical load is shorted.
It is a further object of this invention to determine when the polarization voltage of a battery has stabilized.